This invention relates to a process for liquefying coal or similar solid carbonaceous fuels. More particularly, this invention relates to an improved process-derived coal liquefaction solvent mixture predominantly comprising cis-decalin.
A large proportion, e.g. about 85%, of the world's estimated fossil fuel resources are in the form of solid carbonaceous fuels. Coal constitutes most of the recoverable deposits of solid carbonaceous fuels, e.g. about 80%. Oil shale and tar sands substantially comprise the remainder.
There are many well-known processes for the direct liquefaction of solid coal to produce light and heavy oils. These processes provide for the reduction of the molecular weight of the coal molecule while increasing the hydrogen content. The resulting liquids are then separated from the ash, e.g. mineral matter, and other unreactive solids.
The more important direct coal liquefaction processes today are basically variations of the same process using a process-derived solvent boiling in the 350.degree. to 1000.degree. F. range. In the Solvent Refined Coal Process (SRC) mineral matter from the coal is thought to catalyze a very mild hydrogenation of the process solvent in the course of the liquefaction reaction. Since the concentration of hydrogen donor species is believed to be minimal in this solvent, stabilization of the coal-free radicals would also occur through the presence of hydroaromatic structures in the coal itself (autostabilization), solvent hydrogen shuttling and the presence of dissolved molecular hydrogen in the reaction zone. Due to the inefficiency of radical stabilization by these methods, liquid yields are minimal. Higher liquid yields are obtained in another donor solvent process where the solvent has been catalytically hydrotreated in a separate reaction vessel to form appreciable amounts of hydroaromatic compounds in the solvent. For example, reference is made to U.S. Pat. Nos. 3,505,203 and 4,189,371. For a procedure which is supposed to produce even greater yields by introducing the hydrotreating catalyst into the coal liquefaction reactor, reference is made to U.S. Pat. No. 3,477,941. While the presence of a catalyst throughout liquefaction should theoretically maintain the solvent hydrogen donor capability at a maximum and also help to upgrade any coal liquids formed in the course of the reaction, rapid catalyst deactivation and handling difficulties are difficult problems to overcome.
It has been found unexpectedly that the coal derived liquefaction solvent mixture as produced in the subject process and predominantly comprising cis-decalin is the preferred paraffinic solvent for coal liquefaction.